The contraction is used more than 'what is', but you can't always replace 'what is' with the contraction. For example, you can't say, "What's it?", but you can say "What's that?"

The English words 'what' and 'is', as pronounced in America, contain vowel sounds that don't exist in Spanish. You'll have to hear them pronounced by someone who speaks American English and try to mimic the sounds.

'Is' ends in a 'z' sound that is similar to the sound made when a Spanish 's' precedes a 'b', 'd', 'g', 'l', 'm', 'n', 'r' or 'v'.

In the contracted form, the 's' is NOT pronounced like an English 'z'. It is pronounced like an 's'. So, 'uots' is a better Spanish vocalization than what you wrote above. I assume the 'z' sound that you wrote is the English 'z' sound and not the sound of the Spanish 'z'.